1. Field of the Invention
The present invention relates to a writable reflection-type optical recording medium in which a layer of a recording material which has a changed energy reflection coefficient at a write portion upon data write is formed on a surface on which track guide grooves are formed.
2. Description of the Prior Art
A writable reflection-type optical recording medium for a DRAW (direct read after write) system such as an optical recording disc is known in which various types of data (video or audio analog or digital data) are recorded thereon in the form of pits (holes) or reproduced therefrom by means of a light spot from a light source such as a semiconductor laser. In such an optical recording disc, the provision of track guide grooves for tracking during write operation is very important. If such track guide grooves are not formed, tracking during write operation becomes complex.
In an optical recording disc of melt type wherein data is written by forming pits (holes) in a thin tellurium-based film formed on a surface on which track guide grooves are formed, the depth of the track guide groove is set to be a value 1/8 the wavelength of the read light, which value is optimal for tracking servo control by the push-pull method. An optical recording disc D.sub.0 has, for example, a structure as shown in FIG. 1. More specifically, a thin recording layer 2 is deposited on one (bottom) surface 1a of a glass or acrylic resin substrate 1 on which track guide grooves PG.sub.0 are formed. A resin layer 3 as a protective layer is formed over (below) the recording layer 2. Read light L.sub.0 becomes incident on and is reflected by the recording layer 2 from the side of the other (top) surface 1b of the substrate 1. Thus, the light L.sub.0 is transmitted through the substrate 1 covering (above) the track guide grooves PG.sub.0. Accordingly, the effective depth of the track guide grooves PG.sub.0 becomes 1/n.sub.0 of 1/8 the wavelength of the read light, where n.sub.0 is the refractive index of the substrate 1.
Tracking servo control is performed by detecting a diffraction signal which is obtained based on the depth of the track guide groove so that a spot scans the track.
In this case, the contrast of the write pit during read operation is slightly degraded by a difference between the effective depth of the track guide grooves PG.sub.0 and 1/8 the wavelength of the read light. However, since the contrast of the recording medium itself upon formation of holes in the thin recording layer 2 is great, such degradation in the contrast of the pit during read operation is negligible.
Another type of recording material such as antimony selenium (Sb.sub.2 Se.sub.3) or tellurium oxide TeO.sub.x (where x.div.1) for high-speed write has recently been proposed. The energy reflection coefficient of this type of recording material is increased by a change in the absorption coefficient when phase transformation from the amorphous phase to the crystal phase is effected upon reception of optical energy.
However, when such a type of recording material is used, the contrast of the recording material itself becomes degraded in comparison with a recording material of melt type as mentioned above, thus resulting in read signals of a low S/N ratio.